Ensemble-mean projected increase in maximum five-daily maximum temperatures (Tx5DX), for the (a) 1.5 °C and (b) 2.0 °C HAPPI simulations, relative to the 'present-decade' simulations. Calculations for each model year have been made for the period April to September and results are presented for the full SAEA domain (14°S–47°N, 20°E–114°E). Panels (c) and (d) are the same as for (a) and (b), but this time in units of σ, where σ is calculated for each grid point as the standard deviation of all Tx5DX estimates in the 'present-decade' ensemble. Bottom panels show RR10 estimates for the (e) 1.5 °C- and (f) 2.0 °C-warmer worlds. Black regions denote regions where a one-in-ten year event under a 2.0 °C-warmer world is unprecedented in the 'present-decade' ensemble. Graphic: Harrington and Otto, 2018 / Environmental Research Letters

By Quirin Schiermeier
20 April 2018

(Nature) – Nations such as Bangladesh and Egypt have long known that they will suffer more from climate change than will richer countries, but now researchers have devised a stark way to quantify the inequalities of future threats.

A map of "equivalent impacts", revealed at the annual meeting of the European Geosciences Union (EGU) this month in Vienna, shows that global temperatures would have to rise by a whopping 3 °C before most people in wealthy nations would feel departures from familiar climate conditions equal to those that residents of poorer nations will suffer under moderate warming.

The Paris climate agreement, adopted by 195 countries in 2015, aims to limit the rise in global mean temperature to 1.5–2 °C above pre-industrial levels. The world has already warmed by one degree or so — and since 1900, the mean number of record-dry and record-wet months each year has also increased.

But the effects of global warming are uneven, and poor regions in the tropics and subtropics are thought to be most vulnerable, for several reasons. They have limited financial resources with which to prepare for shifts in temperature and precipitation, and they are expected to face bigger changes in climate than countries in the mid-latitudes. Researchers have had difficulty quantifying those inequalities because the impacts of climate change depend on many factors, such as future economic growth and technological progress, which are hard to forecast.

Luke Harrington, a climate researcher at the University of Oxford, UK, took a different approach by developing the concept of “equivalent impacts”, which doesn’t specify societal consequences. Instead, it focuses on quantifying the uneven distribution of extreme weather around the globe.

Harrington looked at changing patterns of extreme daily heat and rainfall in global climate projections based on fast-rising greenhouse-gas emissions. He then determined how much warming was required for a clear climate-change signal — such as extreme temperatures or precipitation — to emerge from the ‘noise’ of natural climate variability at each spot on the globe. The resulting maps show how quickly regional changes in weather extremes will manifest in response to different levels of global warming.

“I wanted to wrap numbers around the unevenness of impacts,” he says. “Climate-mitigation policies focus on a global threshold — but global mean temperature isn’t a very meaningful metric to assess what climate change might mean in specific parts of the world.”

For changes in regional heat extremes, the pattern is particularly stark. Africa, large parts of India and most of South America are likely to experience changes clearly attributable to climate warming early on, after a 1.5-degree increase in global temperatures. But mid-latitude regions — where most greenhouse gases are produced — won’t see such pronounced changes until the global temperature rise hits 3 degrees or so. [more]

Clear signs of global warming will hit poorer countries first

ABSTRACT: Understanding how continuing increases in global mean temperature will exacerbate societal exposure to extreme weather events is a question of profound importance. However, determining population exposure to the impacts of heat extremes at 1.5 °C and 2 °C of global mean warming requires not only (1) a robust understanding of the physical climate system response, but also consideration of (2) projected changes to overall population size, as well as (3) changes to where people will live in the future. This analysis introduces a new framework, adapted from studies of probabilistic event attribution, to disentangle the relative importance of regional climate emergence and changing population dynamics in the exposure to future heat extremes across multiple densely populated regions in Southern Asia and Eastern Africa (SAEA). Our results reveal that, when population is kept at 2015 levels, exposure to heat considered severe in the present decade across SAEA will increase by a factor of 4.1 (2.4–9.6) and 15.8 (5.0–135) under a 1.5°- and 2.0°-warmer world, respectively. Furthermore, projected population changes by the end of the century under an SSP1 and SSP2 scenario can further exacerbate these changes by a factor of 1.2 (1.0–1.3) and 1.5 (1.3–1.7), respectively. However, a large fraction of this additional risk increase is not related to absolute increases in population, but instead attributed to changes in which regions exhibit continued population growth into the future. Further, this added impact of population redistribution will be twice as significant after 2.0 °C of warming, relative to stabilisation at 1.5 °C, due to the non-linearity of increases in heat exposure. Irrespective of the population scenario considered, continued African population expansion will place more people in locations where emergent changes to future heat extremes are exceptionally severe.

Changing population dynamics and uneven temperature emergence combine to exacerbate regional exposure to heat extremes under 1.5 °C and 2 °C of warming



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